1. Field of the Invention
The present invention relates generally to circuitry for driving an active or passive matrix liquid crystal display (LCD) or the like, and more particularly, to a circuit and method which reduce the amount of power required for driving columns of the LCD display matrix.
2. Description of the Background Art
LCD displays are used today in a variety of products, including hand-held games, hand-held computers, and laptop/notebook computers. These displays are available in both gray-scale (monochrome) and color forms, and are typically arranged as a matrix of intersecting rows and columns. The intersection of each row and column forms a pixel, or dot, the density and/or color of which can be varied in accordance with the voltage applied thereto in order to define the gray shades of the liquid crystal display. These various voltages produce the different shades of color on the display, and are normally referred to as "shades of gray" even when speaking of a color display.
It is known to control the image displayed on the screen by individually selecting one row of the display at a time, and applying control voltages to each column of the selected row. The period during which each such row is selected may be referred to as a "row drive period". This process is carried out for each individual row of the screen; for example, if there are 480 rows in the array, then there are typically 480 row drive periods in one display cycle. After the completion of one display cycle during which each row in the array has been selected, a new display cycle begins, and the process is repeated to refresh and/or update the displayed image. Each pixel of the display is periodically refreshed or updated many times each second, both to refresh the voltage stored at the pixel as well as to reflect any changes in the shade to be displayed by such pixel over time.
LCD displays used in computer screens require a relatively large number of such column driver outputs. Color displays typically require three times as many column drivers as conventional "monochrome" LCD displays; such color displays usually require three columns per pixel, one for each of the three primary colors to be displayed. Thus, a typical VGA (480 rows.times.640 columns) color liquid crystal display includes 640.times.3, or 1,920 column lines which must be driven by a like number of column driver outputs.
The column driver circuitry is typically formed upon monolithic integrated circuits. Assuming that an integrated circuit can be provided with 192 column output drivers, then a color VGA display screen requires 10 of such integrated circuits (10.times.192=1,920). One of the goals of circuit designers is to reduce the power consumption of such integrated circuits, both to minimize power drain on the batteries supplying such power and to reduce the power dissipated within the integrated circuit, and hence reduce the temperature at which such integrated circuit operates.
Integrated circuits which serve as column drivers (or "source drivers") for active matrix LCD displays generate different output voltages to define the various "gray shades" on a liquid crystal display. These varying analog output voltages vary the shade of the color that is displayed at a particular point, or pixel, on the display. The column driver integrated circuit must drive the analog voltages onto the columns of the display matrix in the correct timing sequence. A preferred circuit for generating such analog voltages is described in co-pending U.S. patent application Ser. No. 183,474, filed Jan. 18, 1994, entitled "INTEGRATED CIRCUIT FOR DRIVING LIQUID CRYSTAL DISPLAY USING MULTI-LEVEL D/A CONVERTER" and assigned to the assignee of the present application.
Liquid crystal displays (LCD's) are able to display images because the optical transmission characteristics of liquid crystal material change in accordance with the magnitude of the applied voltage. However, the application of a steady DC voltage to a liquid crystal will, over time, permanently change and degrade its physical properties. For this reason, it is common to drive LCDs using drive techniques which charge each liquid crystal with voltages of alternating polarities relative to a common midpoint voltage value. It should be noted that, in this context, the "voltages of alternating polarities" does not necessarily require the use of driving voltages that are greater than, and less than, ground potential, but simply voltages which are above and below a predetermined median display bias voltage. The application of alternating polarity voltages to the pixels of the display is generally known as inversion.
Thus, driving a pixel of liquid crystal material to a particular gray shade actually involves two voltage pulses of equal magnitude but opposite polarity relative to the median display bias voltage. The driving voltage applied to any given pixel during its row drive period of one display cycle is typically reversed in polarity during its row drive period on the next succeeding display cycle. Thus, for a given pixel located in a particular row, the voltage applied thereto might be +6 volts on a first display cycle and -6 volts on the next display cycle. Over time, the average voltage to which the pixel is driven is a median bias point halfway between the positive and negative voltages; in the example set forth above, the median bias voltage is zero volts or ground.
Assuming that a pixel is initially charged to +6 volts during a first display cycle, then during the following display cycle, the column driver circuit that drives the column which intersects the corresponding row where such pixel is located must drive the pixel from its prior value of +6 volts all the way down to -6 volts, a negative transition of 12 volts. On the third display cycle, the column driver circuit will need to drive the same pixel from -6 volts back to the initial +6 volts (or to some other voltage above the median bias voltage if information is to be updated), a positive transition of as much as 12 volts. The same is true for the other 639 pixels (or the other 1,919 pixels, in the case of a color display) located in the same row, as well as for the pixels located in the other 479 rows. These relatively large voltage transitions result in significant power usage which must be sourced by the column driver circuits.
While the most trivial inversion scheme would be one in which every pixel on the display is first driven to its positive value during a first display cycle, and then driven to its negative value during the second display cycle, this scheme may cause the LCD to alternately display two slightly different images, which could be perceived by the viewer as a flicker in the display. Thus, more complex row inversion schemes are commonly employed to reduce or eliminate any such flickering. Typically, a row inversion technique is used such that, during a display cycle, the driving voltages applied to the columns of the array will alternate in polarity between successive row drive periods. Thus, if the pixels in a first row are driven with positive voltages during the first row drive period, then the pixels in the adjacent second row will be driven with negative voltages during the second row drive period, and so forth. During the next display cycle, the polarities are reversed. Hence, during the second display cycle, the pixels in the first row are driven with negative voltages during the first row drive period, and the pixels in the adjacent second row are driven with positive voltages during the second row drive period, and so forth.
When the row inversion scheme described above is used, a given column driver may, for example, need to establish +6 volts on its associated column during the first row drive period of the first display cycle, and then need to establish -6 volts on the same column during the immediately following row drive period. Thus, the column driver must transition from +6 volts to -6 volts, and back again, for every row drive cycle in every display cycle. These relatively large and frequent voltage transitions consume significant amounts of power.
An even more complex inversion scheme is also known to those skilled in the art whereby the voltage applied to each pixel is of opposite polarity from every other pixel adjacent thereto. In other words, if a pixel is charged with a positive polarity voltage, then the adjacent pixels within the same row are charged with negative polarity voltages, and the adjacent pixels in the same column but in the preceding and following rows are also charged with negative polarity voltages, thus forming a "checkerboard" pattern of voltages. In this checkerboard scheme, column driver integrated circuits are typically disposed at both the top and bottom of the display, and drive alternating columns. For example, in a typical 480 row.times.1920 column display, the odd-numbered columns 1, 3, 5, . . . , 1919 are driven from the column driver I.C.'s at the top of the display, while even-numbered columns 2, 4, 6, . . . , 1920 would be driven from the bottom of the display. Since most known column driver I.C.'s allow for global polarity control (i.e. all outputs will drive high, or all outputs will drive low), then it is straightforward to drive adjacent display columns with opposite polarity driving voltages during a given row drive period by simply inverting a global polarity control signal between the top and bottom column drivers of the display.
The aforementioned global polarity control signal can be alternated between high and low logic levels between successive row drive cycles to invert the polarity of the driving voltage on a given column for every row drive period; thus, during a first row drive period, column 1 may be driven positive, and column 2 may be driven negative, while during the second row drive period, column 1 is driven negative, and column 2 is driven positive. This manner of operation may be viewed as column inversion. If this is done in conjunction with the row inversion technique described above, then the voltage polarity on the pixels of the display will alternate, at any one time, in a "checkerboard" fashion, such that no pixel is driven with the same polarity voltage as any of its neighbors.
For optimum performance, an active matrix liquid crystal display (AMLCD) should be driven with voltages ranging between +/-6 Volts with respect to the median bias point. While this voltage range is certainly attainable with known integrated circuit column drivers, it typically precludes the use of small geometry integrated circuit processes, which only support operation at 5 Volts or less. Since column drivers capable of supplying driving voltages exceeding 5 volts must be fabricated using larger geometry processes, available column driver integrated circuits for driving active matrix displays are typically larger and, therefore, more expensive to produce.
In order to avoid such additional expense, it is known to employ an AC drive technique which allows the use of 5 Volt process technology for fabrication of column driver I.C.'s. This AC drive technique relies upon the column drivers themselves to supply only a portion of the total drive voltage which appears across the liquid crystal pixels. The balance of the voltage across each pixel is supplied by driving the backplane display bias voltage with an AC waveform that is out of phase with the column drivers. Consequently, when the column drivers are outputting a positive polarity voltage, the backplane bias voltage is driven by a negative polarity voltage. The resulting voltage across each liquid crystal pixel is the sum of the voltage generated by the column driver plus the backplane bias voltage.
This AC drive technique generally requires that the polarity of the backplane bias voltage be reversed, and that the polarity of the column drivers also be reversed, following each row drive period. The circuit which drives the backplane bias voltage must switch from, for example, +8 volts to -2 volts between the first and second row drive periods, and from -2 volts back to +8 volts between the second and third row drive periods. In each case, the backplane voltage driver must switch through a transition of ten volts. As the backplane of the display has a significant amount of capacitance associated therewith, a significant amount of power is consumed to continuously switch the backplane bias voltage between successive row drive periods.
Accordingly, it is an object of the present invention to provide a column driver circuit for driving the columns of a liquid crystal display matrix and which reduces the power consumed from a power source when applying alternating polarity drive voltages to the columns of the LCD matrix.
It is another object of the present invention to provide such a circuit which reduces the power dissipated within such circuit when applying alternating polarity drive voltages to the columns of the LCD matrix.
A further object of the present invention is to provide such a power-saving circuit compatible with known row inversion driving schemes for LCD displays.
A still further object of the present invention is to provide such a power-saving circuit compatible with known column inversion driving schemes for LCD displays.
A yet further object of the present invention is to provide such a power-saving circuit for reducing the power consumed by an active matrix LCD display wherein the backplane bias voltage of the display is driven using the AC drive technique described above.
Still another object of the present invention is to provide a method for driving liquid crystal displays which reduces power consumption.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.